TEL AVIV — A powerful bomb, apparently placed by Islamic militants opposed to the Arab-Israeli peace negotiations, blew up a crowded bus during the morning rush hour in the heart of Tel Aviv on Wednesday, killing 22 people and wounding 48.

The explosion lifted the bus off the pavement and scattered charred bodies of passengers--many of them dismembered--up and down Dizengoff Street, turning cosmopolitan Tel Aviv's central thoroughfare into a slaughterhouse.

"People went flying into the air, arms and legs thrown in different directions," said Moshe Reiner, who was walking to work when the blast occurred at 8:55 a.m. "It was a very terrible thing. There are no words, really, to describe the horror we saw."

The death toll was the highest in any terrorist attack in Israel in 16 years.

The Islamic Resistance Movement, known as Hamas, said in Amman, Jordan, and in telephone calls to local radio stations that it was responsible for the bombing, its third and most devastating attack in Israel in two weeks. Hamas opposes the year-old peace accord between Israel and the Palestine Liberation Organization and has vowed to carry its fight into Israeli cities.

In an earlier statement read at noontime prayers in mosques in the Gaza Strip, where Hamas has strong support, the group's military wing known as the Izzidin al-Qassam Brigades declared there would be more attacks.

"God will torment them with your hands and the hands of the faithful," the Al-Qassam statement said, calling the Tel Aviv bombing revenge for the killing of three Hamas members after they kidnaped and subsequently shot dead an Israeli soldier last week. Hamas gunmen had earlier killed two people in Jerusalem's cafe district.

Prime Minister Yitzhak Rabin, who cut short a visit to London and convened an emergency meeting of his security advisers on his return, vowed to hit back hard at Hamas, saying he would seek legislation broadening the powers of Israel's security police to hunt down its leaders, detain them without charge and use harsher methods in interrogating them.

Comments by Rabin and other top Israeli officials indicated that a major roundup of Hamas supporters was planned for the West Bank, in Arab East Jerusalem and, with or without the cooperation of PLO Chairman Yasser Arafat, in the Gaza Strip, where Palestinians have self-rule.

"Ways need to be found so that suiciders, the murderers of Hamas, will know that they are not the only ones who can be killed in their operations, but also their houses, the houses of their families, could be harmed," Rabin said. "Certainly, I cannot find the words to express the pain and anger at the murderous attack against innocent civilians in the heart of Tel Aviv by the Hamas organization."

President Ezer Weizman, visiting the injured in Tel Aviv's Ichilov Hospital, said the Tel Aviv bombing would "require an unusually harsh reaction--different from what has been done so far."

"This cannot be allowed to continue," Weizman said. "We will have to catch (Hamas members), to tear them apart, to chop them to pieces. This is what I'm certain the Israel Defense Forces and the security service will do."

Israel quickly closed its frontiers with the Gaza Strip and the West Bank on Wednesday, barring Palestinians from crossing into Israel to work. And Rabin said that, in the long term, "We need a separation between us and the Palestinians, not just for days but as a way of life."

Rabin declared, however, that Israel will continue to pursue peace with the Palestinians and its other Arab neighbors--but demand that they take tougher actions against Hamas.

"What the Islamic movements are fighting is not the peace process," an angry and exhausted Rabin said, "but the very existence of the state of Israel for they see the peace process securing Israel's existence."

Even victims of the bombing agreed. "The peace process has claimed a heavy price, but I think we have to continue with it and not stop," said Shmulik Sadan, 28, who was on his way to work and reading his newspaper at the back of the bus when the explosion occurred. "Every time, it hurts again. But if we don't make peace, it will just get worse and worse."

Arafat called Foreign Minister Shimon Peres to convey his condolences and offer assistance in finding those responsible for the bombing. Other Palestinian ministers condemned the bombing as brutal terrorism intended to halt the peace process.

"Pushing forward the peace process is a major part of responding to the actions of extremists who receive instructions, training and funding from known external parties," Arafat said in a statement denouncing the attack.

Israel contends the PLO has done too little to prevent violence by the Islamic groups, and Rabin and Peres said they will make "new demands" of Arafat in his ongoing negotiations with Israel to ensure its security.